Show notification on search page if configured

If $wgCirrusSearchShowNowUsing is true then show a note about this wiki
now using CirrusSearch.  $wgCirrusSearchShowNowUsing defaults to false.

The message's phrasing is consistent with similar phrasing in the OAuth
extension.

Bug: 57024
Change-Id: If45aa7309e7a876fa5098a8c9e47450ef9d18f9d
Merge "Show notification on search page if configured"